The present invention relates to pattern inspection systems, and more particularly to a system for inspecting defects of a pattern of a land portion provided around a through-hole on a printed board.
In order to meet the recent requirement for heightening the mounting density of electronic parts on a printed board, improvement for fining a wiring pattern is being made increasingly. Because difficulty is encountered to keep a high accuracy by the conventional human-eye-based inspection system, it is strongly desired to automatically perform the wiring-pattern inspection. One known approach is a defect detecting system such as is disclosed in "Machine vision techniques for inspection of printed wiring boards and thick-film circuits", J. Opt. Soc. Amer., vol 3, no. 9, pp. 1465-1482, September 1986, written by J. L. C. Sanz and A. K. Jain. The conventional wiring pattern inspection techniques are generally classified mainly into the so-called design-rule system and comparison system. Of these conventional wiring pattern inspection techniques, there is a promising system which is arranged to detect a defect of a wiring pattern by contracting or expanding bi-level image data before performing the thinning process. This system will be described hereinbelow with reference to FIG. 1 where (a) to (d) are illustrations for the procedure of detection of shorting defect and (e) to (h) are illustrations of the procedure of detection of projecting defect. First, in (a) of FIG. 1, a point a represents a non-defective portion, and points b and c respectively denote fatally defective portions accompanying the possibility of line-width abnormality or disconnection. In a first step, a contraction process (erosion process) of the image is performed so that a defect appears at the point b as disconnection as illustrated in (b) of FIG. 1. This contraction process is for scraping the image by one pixel from its circumferential portions to overstate defects of the pattern, thereby causing the defect to appear as the disconnection. A second step is then performed so as to effect the thinning process of the pattern up to one pixel width as shown in (c) of FIG. 1. This thinning process is for repeatedly scraping the image by one pixel from its circumferential portions so that the width of the pattern is fined to cause the pattern to be shown by fine lines (having one pixel width, for example). Thereafter, a third step is for deciding that the 3.times.3 local areas (indicated by the square boxes) are in the disconnected states as illustrated in (d) of FIG. 1, thereby detecting the disconnections of the points b and c. At this time, the states of the junction points (indicated by the circles) between the terminal portions and the wiring pattern are also detectable. This defect detecting process is effected by scanning 3.times.3 logical masks and by referring to look-up tables.
Furthermore, a description will be made in terms of defects due to projections. In (e) of FIG. 1, let it be assumed that the points b and c respectively show fatally defective portions accompanying the possibility of the line-width abnormality and short and the point a is a non-defective portion. A first step is executed so as to perform the expansion process by a predetermined size to generate a new connected state at the point b as illustrated in (f) of FIG. 1. Generally, the expansion process is for expanding the image by one pixel toward the outside, thereby causing the point b to enter into the short state. Subsequently, the thinning process is performed as a second step so as to fine the pattern as shown in (g) of FIG. 1. Thereafter, the 3.times.3 logical mask scanning process is executed as a third step so as to decide that the 3.times.3 local areas (indicated by the square boxes) the line-fined image are in the connected states as illustrated in (h) of FIG. 1, thereby detecting the shorts at the points b and c. With this process, the states of the junction points (indicated by circles) between the terminal portions and the wiring pattern are also detectable. Here, the technique of the image process such as the thinning process, expansion process and contraction process is generally known as exemplified by "Elements of Image Recognition" published by Ohm Sha.
There is a problem which arises with such a wiring pattern inspection system, however, in that, in the case of inspecting a printed board having through-holes, there is the possibility that the pattern of the land portion (provided around a through-hole and electrically coupled to a wiring pattern on the printed board) is cut so that the under-width of the line (insufficiency of the line-width) of the land portion can be detected in error when the contractions for both the conductive portion and land portion are made under the same condition. This is caused by the fact that the pattern-width references of the conductive portion and the land portion are different from each other.